The present invention relates to an image reading method including shading correction and device therefor, and more particularly to an image reading method including peak-hold-type shading correction and device therefor.
Generally, image readers (IRs) for optically reading document images suffers luminous energy distortion i.e. shading in photoelectric-converted outputs from image sensors due to a luminescence property of a light source, an optical imaging characteristic, and ununiform sensitivity of image sensors such as CCD (Charge Coupled Device) linear sensors. Electronic correction of the shading is mainly achieved through an average value type method and a peak hold type method that are disclosed for example in Japanese Patent Laid-Open Publication No. HEI 10-271328.
In the average-value-type shading correction, image data for a plurality of lines on a shading plate (white plate) is read in advance, then an average value of the image data is calculated per set of corresponding pixels on each line as shading correction reference data, and upon reading of documents, document image data is corrected based on the correction reference data. The average-value-type shading correction is not easily effected by the influence of S/N (signal to noise) ratio of a system, but susceptible to “dirt” attached to the shading plates. In addition, the average-value-type shading correction requires a memory having a large capacity.
In the peak-hold-type shading correction, on the other hand, image data for a plurality of lines on a shading plate is read in advance, then a peak value of the image data is held per set of corresponding pixels on each line as shading correction reference data, and upon reading of documents, document image data is corrected based on the correction reference data. The peak-hold-type shading correction is not susceptible to “dirt” present on the shading plate and it does not require a large capacity of memory. However, the peak-hold-type shading correction is easily effected by the influence of S/N ratio.
Generally, since sufficient exposure was obtainable in the past, S/N ratio of a system was satisfactorily large and a difference “d” between a peak value “p” and an average value “a” upon reading was small as shown in FIG. 12A. Therefore, the peak hold method was used without any problem.
In recent years, however, it is difficult for digital copying machines and the like to obtain sufficient exposure from image readers because of demands for high CPM (copied pages per min.) and low power consumption. In addition, high resolution and high CPM increase clock frequencies, which generates larger noises in a GND (ground) line, resulting in insufficient voltage of analog image signals outputted by CCD sensors. Because of these reasons, S/N ratio of a system is getting small and a difference d′ between a read peak value p′ and an average value a′ is increasing as shown in FIG. 12B. This increases a peak hold value (shading correction reference data) p′, with which an amplification coefficient of the shading correction is decreased. For example, in the case where a white color in the same level as the shading plate is corrected to a step value of 255, the amplification coefficient is decreased from A to A′. As a result, an intensity level of the entire image is degraded, which causes a so-called “low-key” tendency. In the case of color copying machines, there arises a problem of change in shades of colors. More particularly, a sensitivity of color copying machines is generally different by RGB (red green blue), and therefore S/N ratio and a S/N ratio fluctuation amount are different per RGB. The color copying machines are usually tuned in such a way that identical RGB readings are obtained when a gray patch is read in a standard S/N ratio condition. However, the S/N ratio is different per RGB, which causes a phenomenon that a certain wavelength element is displaced in low-key direction whereas another wavelength element is displaced in high-key direction. As a result, dispersion is generated in readings between RGB, which causes change in shades of colors, that is, a color tone regardless of a gray patch.
In a system having inappropriate S/N ratio, the dispersion in the S/N ratio among individual machines is also large. Consequently, the S/N ratio causes larger dispersion in readings of an document image after application of shading correction, which makes the above-stated problems aggravated.